Ouro Fino Tourmaline
Ouro Fino Tourmaline
Elbaite from a small Minas Gerais pegmatite district
Ouro Fino tourmaline is elbaite produced from the pegmatites of the Ouro Fino district in the south of Minas Gerais state, Brazil. The locality has supplied the cutting market since the early twentieth century, and although its output is small relative to the great Brazilian pegmatite belts farther north, Ouro Fino has a well-established reputation in the trade for clean, well-saturated pink and green tourmaline and for bicolour material in which a single crystal carries adjacent zones of contrasting hue.
Geological setting
The Ouro Fino district lies near the southern margin of the broader pegmatite province of Minas Gerais, the same province that hosts the Governador Valadares, Araçuaí, and São José da Safira fields to the north and northeast. Tourmaline mineralisation is in granitic pegmatites intruded into a Proterozoic metamorphic basement, with elbaite crystallising in the late, lithium-rich miarolitic pockets along with quartz, lepidolite, and the cleavelandite habit of albite. The pockets are typically small and discontinuous, and production is sporadic by the standards of the larger Brazilian deposits.
Colour range and material character
Ouro Fino crystals run principally in pink and green elbaite and in pink-and-green bicolour and tricolour combinations, with the colour zones lying along the c-axis of the prism. Saturation is usually moderate to high, with the pinks tending toward a clean, slightly bluish rose and the greens running from a leek-green through to a more saturated grass-green. The crystals are characteristically eye-clean, with the inclusion suite typical of pegmatite elbaite — fine fluid-filled tubes parallel to the c-axis, occasional needles, and infrequent cleavelandite intergrowth.
Cut sizes from the locality are small to medium. Faceted stones above ten carats are uncommon, and the great majority of commercial Ouro Fino material trades in the one-to-five-carat range. The cutter typically orients the rough to optimise colour along the table, accepting the c-axis darkening that is normal for tourmaline.
In the trade
Ouro Fino is one of several Minas Gerais provenance terms used in the coloured-stone trade. It does not command the price premium associated with Paraíba — the cuprian elbaite from the Batalha and São José da Batalha mines farther north — but is recognised as a reliable source of clean commercial-to-fine elbaite. Origin in the Ouro Fino sense is rarely a determinant of value; buyers price the stone on colour, clarity, and size, and use the locality name principally as a sourcing reference. Production from the district has been irregular, and parcels offered as Ouro Fino are often blended with material from neighbouring Minas Gerais workings.
Treatment and identification
Heat treatment is occasionally applied to Ouro Fino tourmaline to lighten over-dark green stones or to shift the orange or brown component out of certain pinks, and irradiation has been used historically to deepen pink saturation. The treatments are not always disclosed in routine commercial parcels and should be assumed unless a stone has been independently certified as untreated. Routine gemmological identification of elbaite from Ouro Fino does not differ from elbaite identification generally — refractive indices around 1.62 to 1.64, biaxial negative optic character, hardness 7 to 7.5, and a specific gravity around 3.05 — and origin determination is not commercially performed for tourmaline at this provenance level.
Care
Ouro Fino tourmaline is durable enough for most jewellery applications, including ring use in protected settings. Pyroelectric and piezoelectric properties of the species mean that abrupt thermal changes should be avoided, particularly in stones with significant inclusion content. Cleansing by warm soapy water is appropriate; ultrasonic and steam cleaning are not recommended for stones with visible inclusions or bicolour zoning where stress at the colour boundary may concentrate.